The present invention generally relates to vehicle brakes, and more particularly relates to a method and a device for controlling traction slip.
When driving on very rough or slippery road surfaces, high traction slip values may occur at the wheels of a vehicle even if the driver accelerates only slightly, that means, the engine has only a low excess torque or traction torque. On rough surfaces this is due to the fact that single wheels will lose ground contact at least temporarily in part or in total. On slippery surfaces, the coefficient of friction between the roadway and one or more wheels may be so low that even a low engine torque will cause spinning of the wheels.
When traction control intervenes in such situations, i.e., the control of tractive force which acts by way of an active pressure increase at the wheel brake circuits (BTCS=Brake Traction Control System), stalling of the engine may be caused when the vehicle is equipped with a manually operated transmission and the driver has engaged the clutch. Such a style of driving is conventional even at low vehicle speeds when the driver recognizes that the wheels generally tend to spin. More particularly, full engagement of the clutch is appropriate in offroad driving when the vehicle has a countershaft transmission and there is an extreme gearing-down in a low off-highway gear.
In general, brake-induced xe2x80x98stalling situationsxe2x80x99 can be avoided because a permanent monitoring of the engine speed, for example, is carried out during traction control. When e.g. the engine speed falls below a critical threshold (xe2x80x98stalling speedxe2x80x99), the risk of stalling of the engine is detected. In the case of such a stalling risk, the active brake pressure increase on the wheels will be stopped, and pressure decrease can be effected with the maximum possible decrease gradient in order to relieve the engine.
Pressure decrease is usually stopped when the engine speed has reached values again which confirm a stabilization of the engine run.
A measure of this type prevents direct stalling of the engine in the majority of cases. It is disadvantageous that the entire control mostly acts very abruptly and causes frequent stuttering of the engine. On difficult off-road tracks, a driver may become disconcerted by such an abrupt intervention and may be induced to even reduce opening of the throttle, which may further impair the performance of the control. The less the throttle is opened, the greater the risk of stalling the engine in the case of an active braking. In addition, complete brake evacuation may lead to unexpected vehicle reactions, such as a sudden rolling back on a hill.
FIG. 1 shows a conventional BTCS control with the example of a spinning wheel.
In this illustration, reference numeral 10 designates the speed variation of the wheel that tends to spin, reference numeral 11 designates the vehicle speed or a substitute value estimated within the controller, reference numeral 12 is assigned to the engine speed, 13 to a speed threshold, and 14 designates the variation of the pressure which the BTCS delivers into the associated wheel brake circuit. Signals 15 and 16 show two speed thresholds which are calculated during the BTCS control on the basis of slip values (in percent) and determine the change-over between different control states.
The control starts with a pressure increase at the point of time T1 when the speed 10 of the spinning wheel has exceeded the higher speed threshold 15. The subsequent pulsed pressure increase ends at the point of time T2 when the wheel speed 10 has fallen below a lower threshold 16. Then a normally pulsed pressure decrease will start. Directly afterwards (point of time T3) the engine speed drops below the speed threshold 13 in the embodiment shown which triggers a pressure decrease with the maximum gradient (unpulsed) according to the conventional control strategy. This steep pressure decrease is required to prevent xe2x80x98stallingxe2x80x99 of the engine in time.
After the engine speed has increased again, the pressure increase after the point of time T4 will be dictated by the slip of the greatly spinning wheel again and will be performed in a relatively steep fashion which, in turn, causes a strong load on the engine and decrease of the rotational speed with a new instability at the point of time T5.
According to the greatly simplified concept in FIG. 1, the change-over between the control states xe2x80x98pressure increasexe2x80x99 and xe2x80x98pressure decreasexe2x80x99 depends on thresholds which, for reasons of clarity, are calculated equidistantly to the (estimated) vehicle speed.
However, the shortcoming of the conventional control is generally seen in that a reaction to the engine in the form of a wheel pressure decrease typically occurs only when the engine speed has dropped below a critical speed threshold. In all other respects, the control and the pressure modulation are only dictated by the behavior of the spinning wheels.
In view of the above, an object of the present invention is to provide a method and a device for controlling traction slip which permit greater engine stability.
According to the present invention, this object is achieved in that a generic method is performed so that at least one further variable which represents a running stability of the engine, is included in the control of the control states and/or the switch-over between the control states.
This renders it possible to make fine adjustments to the traction slip while taking into account the engine situation. The control is dictated by the engine situation which is taken into consideration in each phase of controlling the traction slip. Beside the traction slip (wheel speed and/or wheel acceleration), the running stability of the engine or an engine stability reserve derived therefrom is a controlled variable which is taken into consideration when the correcting variable is formed.
Another object of the present invention is to design a generic device for controlling the traction slip so that a first determination device determines a variable which determines a wheel behavior (speed and/or acceleration) on at least one of the driven wheels and, in dependence on this variable, controls control states such as increase brake pressure, reduce brake pressure, or maintain brake pressure, and regulates the change-over between the control states such a increase brake pressure, reduce brake pressure, or maintain brake pressure, or switch on or off traction slip control, and a second determination device determines at least one further variable which represents a running stability of the engine and makes the first determination device become involved in the control of the control states and/or the change-over between the control states.
To further improve the control behavior, the second variable is continuously considered in the control and/or change-over.
The running stability of the engine is calculated from the difference between an engine speedF-E-S and a dynamic instability threshold of the engine, preferably according to the following relation:
ENGINE_STABILITY_RESERVE=K1xe2x80xa2(FILTERED_ENGINE_SPEEDxe2x88x92ENGINE_STALLING_THR),
with ENGINE_STABILITY_RESERVE=running stability of the engine, K1=a constant dependent on the engine characteristic and on the average vehicle weight, FILTERED_ENGINE_SPEED=the filtered engine speed, and ENGINE_STALLING_THR is the instability threshold.
The running stability of the engine or the engine stability reserve is formed by the difference between the current (filtered) engine speed and a dynamic instability threshold according to the relation mentioned hereinabove in that the dynamic instability threshold was calculated by that a portion is subtracted from a vehicle-related base value which is determined proportionally to the gradient of the engine speed.
The second variable is used for switching over between the pressure-increase and pressure-decrease control phases of the BTCS by initiating a pressure decrease when the second variable falls below a lower threshold value, and a pressure increase is not permitted until the second variable exceeds a higher threshold value.
In the case of pressure increase during traction control, a maximum pressure increase gradient is predetermined by the second variable, and a high running stability of the engine permits a steeper pressure increase, while a low running stability forces a flatter pressure increase.
The decrease and increase gradients are adapted into the engine characteristic, and a higher engine torque renders possible a faster increase and a slower decrease of wheel pressures.
The terms xe2x80x98steeperxe2x80x99 and xe2x80x98flatterxe2x80x99 pressure increase or decrease refer to the pressure modulations which the per se known BTCS control would provide.
In the case of a pressure decrease during traction control, a minimum pressure decrease gradient is predetermined by the second variable, and a high running stability of the engine allows a flat pressure decrease, while a low running stability forces a steeper pressure decrease.
The method according to the present invention ensures even with a very moderate style of driving that a medium pressure level to be mastered by the engine is introduced into the brake circuits of the wheels that tend to spin so that a traction control is achieved which takes into consideration not only the wheel slip situation but also the engine situation so that an optimal compromise is reached. The driver senses this control as being so smooth that xe2x80x98playing with the accelerator pedalxe2x80x99 is possible in difficult situations, whereby the vehicle can be balanced e.g. on a steep, bumpy slope.
This measure can be employed for any types of drives. Even with an automatic transmission which typically prevents stalling of the engine on its own, the mentioned method permits achieving a considerable improvement of the control comfort and the control function.
The method is based on the idea of not only considering the traction slip as a controlled variable in the event of spinning of one wheel but to permanently adapt the cyclic pressure increase and pressure decrease on a wheel to the engine situation.
This means that if a vehicle tends to spin already when the accelerator pedal is only slightly depressed, only a delayed and flat pressure increase is allowed to be performed in the beginning. When the engine speed is reduced by this pressure increase, a pressure stop or even pressure decrease may occur already when the wheel continues to show spinning tendencies and the engine speed has not yet exceeded a threshold which is deemed to be a critical xe2x80x98stalling speedxe2x80x99 for the respective type of engine.
If, however, the engine exhibits again a stabilization in the way of a rising rotational speed, it may be subjected to being newly loaded due to pressure increase. However, pressure increase is not dictated by the degree of the wheel spin, i.e., by the wheel slip, but by the xe2x80x98stabilityxe2x80x99 of the engine. Only if the engine is operated at high speed will traction control pass over into a pure wheel slip control.
This type of control is advantageous because the engine, from the very beginning, is only loaded to such an extent that an instability tendency is unlikely to occur. This avoids the development of high pressure peaks which would always have to be removed very quickly to prevent a destabilization of the engine.
The control state and the degree of pressure modulation are provided in dependence on a defined engine stability reserve. The latter, in turn, is calculated from the difference between the current engine speed and an instability threshold.
With a small stability reserve, there will be an early switch-over to the control state xe2x80x98maintaining of pressurexe2x80x99 or even xe2x80x98pressure decreasexe2x80x99.
On the contrary, the control state xe2x80x98pressure increasexe2x80x99 may only be activated when the stability reserve has exceeded a high threshold value.
In the control state xe2x80x98pressure increasexe2x80x99, the pressure increase gradient is calculated as a function of the stability reserve by building up the pressure with a lower gradient in the event of a small reserve.
In the control state xe2x80x98pressure decreasexe2x80x99, the pressure decrease gradient is also calculated as a function of the stability reserve by reducing the pressure with a higher gradient in the event of a small reserve.
When the value of the stability reserve amounts to zero, pressure decrease with the hydraulically possible maximum gradient will occur.